1. Field of the Invention
The present invention relates to a liquid crystal display device for color display and a method for fabricating the same.
2. Description of the Related Art
FIG. 12 is a schematic view of a liquid crystal display device for color display. Referring to FIG. 12, source signal lines 101 and gate signal lines 102 run crossing each other, with pixels 103 formed at the respective crossings to be arranged in a matrix. Each of the pixels 103 includes a thin film transistor (TFT) 104 as well as a pixel capacitance 105 and a storage capacitance 106 connected to a drain of the TFT 104. The pixel capacitances 105 of the pixels 103 are connected to a counter electrode (not shown) of the liquid crystal display device via capacitance lines 107.
Sources of the TFTs 104 of each vertical column are commonly connected to one source signal line 101. Gates of the TFTs 104 of each horizontal row are commonly connected to one gate signal line 102.
The gate signal lines 102 are sequentially made active in synchronization with each scanning, thereby turning on the corresponding horizontal rows of TFTs 104. Every time one row of TFTs 104 are turned on, video signals allocated to the source signal lines 101 are allowed to be written in the pixel capacitances 105 of the TFTs 104 of the row. In this way, a set of video signals representing one image on a display screen are written in all the pixel capacitances 105 arranged in a matrix, thus accomplishing the display of the image.
FIG. 13 is an enlarged view of one pixel of the conventional liquid crystal display device, and FIG. 14 is a sectional view taken along line C-C' of FIG. 13.
Referring to FIG. 13, each pixel of the liquid crystal display device includes the TFT 104, a storage capacitance electrode 113, and a rectangular pixel electrode 114. The gate signal lines 102 run along the upper and lower peripheries of the pixel electrode 114, and the source signal lines 101 run along the right and left peripheries of the pixel electrode 114.
Referring to FIG. 14, the gate signal line 102 and the storage capacitance electrode 113 (not shown in FIG. 14) are formed on a substrate 116, and a gate insulating film 117 is formed over the resultant substrate. A semiconductor layer 118 and a channel protection layer 119 are then formed, followed by the formation of an n.sup.+ -Si layer 120 which is to be the source and drain of the TFT 104. An ITO film is then formed and patterned to form a drain signal line 112 and the source signal line 101. An interlayer insulating film 121 is formed over the resultant substrate, and a contact hole 122 is formed through the interlayer insulating film 121. Another ITO film is formed on the interlayer insulating film 121 and patterned, to form the pixel electrode 114 so that the pixel electrode 114 is connected with the drain signal line 112 via the contact hole 122. An alignment film 123 is formed on the pixel electrode 114 and rubbed. Thus, an active matrix substrate 131 is formed.
A photosensitive color resist film 126, a counter electrode 127, and an alignment film 128 are sequentially formed in this order on a substrate 125, so as to form a counter substrate 132.
The resultant active matrix substrate 131 and the resultant counter substrate 132 are placed to face each other, and liquid crystal is injected therebetween, to form a liquid crystal layer 124.
With the above configuration where the interlayer insulating film 121 is formed above the source signal line 101 and the gate signal line 102 to separate these lines from the pixel electrode 114, the pixel electrode 114 is allowed to overlap the signal lines 101 and 102. This enhances the aperture ratio of the pixel, and also blocks the electric field caused by the signal lines, thereby suppressing a failure in the orientation of liquid crystal molecules (see Japanese Laid-Open Publication No. 58-172685).
Alternatively, the photosensitive color resist film 126 of the counter substrate 132 may be omitted and, instead, a layer of a black mask and a color filter may be formed integrally in the active matrix substrate 131 to serve as the interlayer insulating film 121 (see Japanese Laid-Open Publication No. 6-242433). This alternative case eliminates the necessity of considering an alignment error at the attachment of the active matrix substrate 131 and the counter substrate 132 which is required in the former case where the black mask and the color filter are formed in the counter substrate 132. This also improves the aperture ratio.
In general, methods such as dying, electrodeposition, and pigment dispersion are employed to form a color filter for each pixel of the liquid crystal display device. In the dying method and the electrodeposition method, the resultant color filter is poor in color fading resistance, or easily prone to color fading. In the pigment dispersion method where a material is applied by spinning to form a film, the material tends to be wasted. This requires further improvement in consideration of the cost reduction of the color filter.
The above problems also apply to the case of forming the black mask/color filter in the active matrix substrate 131 as the interlayer insulating film 121. Moreover, this method has additional problems as follows. The potential written in the entire capacitance of each pixel (the sum of the pixel capacitance and the storage capacitance) needs to be held for a predetermined time period (substantially corresponding to a frame period). This requires the TFT 104 to be covered with the black matrix to suppress the photoconductance of the TFTs 104. However, the characteristics of the TFTs 104 provided by the interlayer insulating film 121 should not be lost by the black matrix formed as the interlayer insulating film 121. Accordingly, the interlayer insulating film 121 serving as the black mask is required to have strict performance in the insulation and non-polarization properties.
Thus, in the case of forming the black mask/color filter in the active matrix substrate 131 as the interlayer insulating film 121, the dying method and the electrodeposition method are further difficult to be employed. When the pigment dispersion method is employed, care must be taken for the selection of the pigment and against pollution of the active matrix substrate with the pigment.
Another problem is as follows. Since the periphery of each pixel electrode 114 overlaps the corresponding gate signal line 102, the potential at the pixel capacitance largely changes due to an influence of a capacitance generated between the pixel electrode 114 and the gate signal line 102 in the overlap portion. When the film is formed by spinning in the pigment dispersion method, the control of the thickness of the interlayer insulating film 121 is difficult. This becomes more difficult the larger the active matrix substrate 131 is (i.e., the display screen is larger), causing a variation in the thickness of the interlayer insulating film 121. If the thickness of the interlayer insulating film 121 is not sufficiently uniform, the change of the potential at the pixel capacitance due to an influence of the capacitance between the periphery of the pixel electrode 114 and the gate signal line 102 is not uniform for all pixels. As a result, a large DC component is applied to a certain portion of the liquid crystal layer, and this significantly lowers the display quality and reliability at the portion.
In yet another problem, since the periphery of each pixel electrode 114 overlaps the corresponding source signal line 101, the capacitance between the pixel electrode 114 and the source signal line 101 becomes large. This causes a video signal on the source signal line 101 to influence the potential at the pixel capacitance via the enlarged capacitance between the pixel electrode 114 and the source signal line 101, varying the potential at the pixel capacitance. In other words, crosstalk arises along the source signal line 101, and thus line noise appears on the display screen.
To summarize the above, in the case of forming the black mask/color filter as the interlayer insulating film 121, strict performance in the insulation and non-polarization properties is required for the interlayer insulating film 121. This makes it extremely difficult to employ the dying method and the electrodeposition method. The pigment dispersion method, on the other hand, has a problem that the control of the thickness of the interlayer insulating film 121 is difficult. This difficulty in the thickness control causes various restrictions.
From the foregoing, the purpose of the present invention is to provide a liquid crystal display device capable of forming a black mask/color filter as the interlayer insulating film without lowering the display quality, and a method for fabricating such a liquid crystal display device.